1. Field of the Invention
The present invention relates to a surface acoustic wave device having a structure which suppresses a wavefront distortion and a disturbing reflected signal produced in an interdigital transducer having a pair of comb-like electrode fingers overlapped with each other and arranged on a piezoelectric substrate.
2. Description of the Prior Art
As shown in FIG. 1, a surface acoustic wave device generally comprises a piezoelectric substrate 1, an input interdigital transducer 2 having a pair of comb-like electrode fingers overlapped with each other for converting an electric signal to a surface acoustic wave, a propagation path 3 along which the surface acoustic wave propagates and an output interdigital transducer 4 having a similar structure to that of the input interdigital transducer 2 for converting the propagated surface acoustic wave to an electric signal. In such a surface acoustic wave device, in order to attain a complex frequency characteristic, the input interdigital transducer 2 and/or the output interdigital transducer 4 is weighted in terms of the overlap length of the two comb-like electrode fingers as shown in FIG. 2. The resulting envelopes denoted by 5a and 5b created by the weighting should be in accordance with a Fourier transform of a desired frequency characteristic. In the interdigital transducer weighted as shown in FIG. 2, it is a common practice to provide dummy electrodes 6 externally of the envelopes 5a and 5b in order to eliminate wavefront distortion of the propageted surface acoustic wave, as is disclosed in U.S. Pat. No. 3,699,364.
Furthermore, in order to eliminate the reflection at the electrode fingers, a split-connected interdigital transducer as shown in U.S. Pat. No. 3,727,155 or an unbalanced double electrode described in an article in "1976 Ultrasonics Symposium Proceedings" by K. Hanma, who is one of the inventors of the present invention, is used. The former interdigital transducer has an electrode finger width equal to .lambda./8, where .lambda. is the wavelength of the surface acoustic wave and the latter interdigital transducer has electrode finger widths equal to .lambda./16 and 3.lambda./16. Thus, when the interdigital transudcer is used as a 50 MHz television intermediate frequency filter, a very narrow electrode of 10 .mu.m or less is required. This has been a cause for poor yield of the device. Furthermore, because of very narrow electrode fingers, the breakage of electrode fingers occurs and the wavefront distortion of the surface acoustic wave occurs because the ratio of the width of the electrode finger external of the envelopes and the gap at which no electrode is disposed is not uniform.